1. Preheat oven to 400°.
2. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and salt in a large bowl; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Cover and chill flour mixture 10 minutes.
3. Combine buttermilk and next 4 ingredients (through egg), stirring with a whisk until well blended. Stir in cranberries and nuts. Add buttermilk mixture to flour mixture; stir just until dough is moist.
4. Turn dough out onto a lightly floured surface. Pat dough into an 8-inch circle on a baking sheet lined with parchment paper. Cut dough into 12 wedges (do not separate the wedges). Brush egg white over dough; sprinkle evenly with turbinado sugar. Bake at 400° for 13 minutes or until golden. Remove from pan; cool 2 minutes on wire racks. Serve warm or at room temperature.